For most crops, 30% risk is probably too much, but this is for you decide.

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1 Frost Probabilities Compiled by Dustin Blakey Farm Advisor, Inyo and Mono Counties Frost and Freeze Dates Every year frost will occur at a different date. It is impossible to know exactly when the last frost will happen or how cold it will get. If one looks at long term data, it is possible to get a general feel for the length of the growing season. We usually do this in terms of the probability of a frost occurring either after a certain date in spring or before a certain date in autumn. Many sources report the average last frost date, but as this number doesn t account for the variability, official frost date estimates usually give dates based on probability. For example, if the average last frost of spring is May 15, the 25% probability would be later, perhaps June 1. This means that there is a 25% chance of a frost occurring on or after June 1. As a gardener or farmer, you decide how much risk you want to take of a crop being exposed to a frost or freeze. A grower of a low value crop that is easy to replant may be willing to take more risk than an orchardist with expensive trees and overhead will be willing to take. If you are too conservative with your risk, you may not have enough growing season to raise a crop. For most crops, 30% risk is probably too much, but this is for you decide. Using These Charts Typically frost predictions are presented using a chart like the one shown below. An example frost chart. Because different growers will be willing to take different levels of risk for their crops, this document uses a chart to visually display frost and freeze risks. Each chart has four lines, each representing the critical date for different temperatures. For simple frost prediction use the 32 F line (blue). Other temperature lines are useful for hardier crops and fruit trees.

2 If you have the ability to protect crops from frost, you can use the line that is appropriate to the level of protection you can provide, or you may choose instead to accept a higher risk level. The vertical axis is the probability and the horizontal represents the date. To use the chart, select a risk level you are comfortable with (perhaps 10% to 25%).The date corresponding with that probability for a given temperature line shows the corresponding date. On the chart below a 0.10 (10%) probability corresponds to a date of June 18. This means there is a 10% probability of a spring frost occurring on or after June 18. Example Chart Fall works the same way as spring except that the probability is of a frost occurring before this date. The span between dates (in the middle of the U shape) for a chosen probability gives the length of the growing season for that risk level. As you accept more risk, the season lengthens. Each vertical line is 28 days apart. Probabilities are calculated using method in Snider & de Melo-Abrieu (2005) Fundamentals of Frost Protection, Vol. 2. Food and Agriculture Organization. National Weather Service data used. Microclimates These frost charts do not account for microclimate variation. They use historical data from National Weather Service stations. Higher elevations and low pockets in valleys will have shorter growing seasons and later freeze events. You should always use common sense and not strictly rely on these estimates. Neighbors can be a good source of advice on how a microclimate may vary from the prediction, but keep in mind human memory can be unreliable. Low Desert Locations We have long-term weather data for Death Valley National Park, but because the climate there is so mild during winter, the data for temperatures of 28 and 26 F are not reliable for predicting freeze dates since many years never get this cold. The data for 30 is better but not perfect. The prediction for 32 is excellent. Included for Death Valley and southeast Inyo Co. residents are probability charts for Death Valley and Amargosa Valley provided by WRCC. Frost is probably not your major concern if you garden in this area.

3 Topaz Lake Station The Topaz Lake station is the best predictor we have of frost/freeze events in Antelope Valley. The station has been moved in the past. Data is from 1958 to 1985 and from 2001 to Elevation, distance from the lake and microclimates will affect this estimate. Antelope Valley has the most highly variable frost season in the Eastern Sierra. To garden here, as a practical matter, you must accept a higher level of risk and be prepared to protect your plants.

4 Bridgeport Bridgeport has weather data from 1932 to 1947 and 1951 to 2011 used in this prediction. There is no point in the year where Bridgeport is not subject to frost. Hard freezes are possible year-round but are rare. This area is very cold and protection is needed to grow most vegetables.

5 Lee Vining There are weather observations for Lee Vining from 1988 to While this is less than we would like to have as a minimum, it is a good place to start. There is a lot of variability to the frost data at this location. Over time we could expect greater accuracy, but there shouldn t be a dramatic difference from what is shown.

6 Mammoth Lakes Mammoth Lakes is a difficult place to predict frosts. We have data from 1994 to This is too few years data to make good predictions, but it may be better than nothing. Use with caution. Because the community is so variable in elevation and microclimate one can expect great variability. Unlike Owens Valley, when spring frost season ends, it really ends; however, this happens fairly late. Early fall frosts are much more variable. The growing season here is very short.

7 Bishop Bishop has a weather station at the airport with temperature data used in this chart from 1948 to This chart is probably conservative in its estimates for the central part of town. It will be fairly accurate for Laws. Chalfant and Big Pine will probably have a shorter frost-free season. While it s the closest station to Round Valley, Rovana and Starlite, it won t be very reliable.

8 Independence There is excellent data for Independence. Except for a few missing years, we have records from 1925 to This is probably the best data set to use for Lone Pine as well.

9 Haiwee Reservoir There is a weather station adjacent to Haiwee Reservoir with data from While the data for this site is fairly reliable, the usefulness may be questionable. Bodies of water tend to add some protection from frost and there is not much population nearby. Gardens in the area will almost certainly have a frost season that extends later in the spring. The data for 26 and 28 are less reliable since there are fewer instances in late winter and spring where the temperature gets that cold than most other sites.

10 Death Valley Charts shown are provided by the Western Regional Climate Center for Furnace Creek and are slightly different as they break up spring and fall predictions into two charts. While the frost-free season is very long, most of that time is unbearably hot. Generally most garden crops will be grown during the potential frost season since it is cooler. Generally frost season is between December and March. Spring Frost Risk Fall Frost Risk

11 Southeast Inyo County There is a weather station in Shoshone, however its long-term data is for precipitation only. Perhaps an estimate somewhere between Death Valley and Amargosa may be close. There is a weather station in Trona, but I m not sure of its usefulness for Tecopa and Shoshone. As a rule, be ready to protect your plants from frost in winter. Amargosa Valley These charts, like Death Valley s, are divided into separate graphs. This station can get colder than many SE Inyo County locations. Spring Frost Risk Fall Frost Risk

12 Darwin Along with convenient access to most modern technology including an internet connection fast enough to download this document Darwin lacks a weather station. Therefore, it s impossible to make reliable predictions of frost for the community. A reasonable estimate is to use the Wildrose Ranger Station s data. This station is at a similar elevation and latitude. While it s not perfect, it is better than flipping a coin. Good luck. Spring Frost Risk Fall Frost Risk September, 2013